Proposed heartbeat implementation

This pull request also includes https://github.com/fusesource/stompjms/pull/13 (now closed)

The proposed implementation implements the STOMP 1.2 spec(http://stomp.github.io/stomp-specification-1.2.html#Heart-beating), and is configurable via the system property:

        -Dstompjms.heartbeat=X,Y

It may be debated if this should be configurable via the ConnectionFactory (and in which form: as a string  or separate values).

Also changed the ActiveMQ integration test to use annotations in order to disable the AMQ-4493 proof (https://github.com/fusesource/stompjms/pull/13).

The proposed implementation has been tested using a camel app as client to ActiveMQ 5.8.0, using stomp-jms.
Restarting broker and client works as a charm, and stopping the heart-beats in the client (via debugger) makes the broker discard the client.

BR. Jan-Helge